Thompson Falls, MT to Seattle, WA Road Trip
 

We are traveling to Thompson Falls, MT this summer to spend a couple of days with friends. We will probably be flying into Glacier airport and seeing the sites at Glacier. We will be leaving and driving to either Seattle or Portland and on down to San Francisco (I have posted another forum for our WA to CA leg of the trip. Looking for anything interesting or fun to stop and see, between MT and WA.

A few years ago we drove through that area and took the Northern Cascade highway which comes into the coast at Anacortes WA. It was amazing!!

Seattle is a great city and Portland is one of my favorite places on earth. So if you were to drive over the Cascade highway, south to Seattle, freeway down to Portland and then cut over to the Oregon coast for a bit and on to San Francisco. Canon beach is very pretty as is Depot Bay and Bandon...then if you keep going south the redwoods are amazing.

By Glacier airport do you mean Kalispell? Thompson Falls is over 100 miles from Glacier NP. I doubt that you will find a rental car company that wants to rent one way to another state.
See Thompson Falls and Glacier then turn in the car and ride the Amtrak Empire builder from Whitefish to Seattle (overnight). Fly or take the train from Seattle to San Francisco.
The rental car company I used from Kalispell airport let me leave the car at the Whitefish Amtrak station.

Your 1-way rental may be more expensive than tomfuller's option of the Empire Builder. It's a nice ride with lots to see from the train.

If you must drive State Route 200 out of Thompson Falls would connect you to US 2 in Sandpoint, ID. Lake Pend Orille is a beautiful lake, mountains all around. Good fishing if that interests you. Nice local museum. A busy rail hub for train watcher,
if you stay on US 2 to rather than taking I-90 after Spokane, you can visit Grand Coulee Dam with a little detour.
Then you decide on North Cascades Highway via the scenic Methow Valley, Twisp and Winthrop or you stay on US 2 tp the heart of Washington Apple Country, Wenatchee and then cross theCascades via Leavenworth and Stevens Pass into the Puget Sound area.
I've tried both routes and think the North Cascades is more scenic, but there are more towns with options along US 2.

Besides being CONSTANLY late, sometimes for HOURS, the train from Whitefish to Seattle travels at night. Tough to see things in the dark.